Physics Department

My Watch is Slow

My Watch Is Slow!

Pocket watches are often kept on a chain in a pocket. If left to dangle, as one walks along, the watch will begin to swing back and forth. Careful observers have said that when this happens, the watch may no longer be accurate, and may lose or gain 10 to 15 minutes per day. Is this true and why or why not?

A. True. The swinging watch case may have an oscillation frequency near or equal to the frequency of the watch balance wheel. This will result in the balance wheel changing oscillation frequency which effects accuracy.

B. True. The swinging watch case increased the kinetic energy of the watch. According to principles of special relativity, as the kinetic energy (and velocity) increase, the mass of the watch will increase. This increase will throw off the oscillation rate of the balance wheel and make the watch run slower.

C. False, the swinging watch case does not effect the oscillation of the balance wheel at all. The watch will keep accurate time, and the careful observers are wrong.